380 



WATER-TRANSPORT. 



purpose a good fouDclation must be supplied, as in fig. 221, to 

 prevent undermining, and a slope of about one in ten should be 

 given. As great a hindrance to floating as steep banks, are 

 banks which are too flat, as the stream widens-out in such places, 

 and tends to fall-oft' in strength, depth, and rate of current. 

 The gravel and other material brought down from above 

 accumulate in such places, forming shoals which the floating 



timber only passes with difli- 

 ^^°- ^^^- culty, and many logs become 



stranded. Improvements at 

 such places have for their 

 object to restrict the bed of 

 the stream. 



The simplest method is to 

 drive-in a double row of piles 

 as close to one another as 

 the length of the logs which 

 are floated, they mark-out 

 the stronger water from the 

 slack water near either bank. 

 The piles are high enough to 

 overtop the highest level attainable by the water, and the logs as 

 they float down align the piles and exclude the dead water. 



In some cases wattle-work is applied round the piles, other 

 rows of piles are driven-in a few feet from the first rows, and 

 the interval filled with stones, branches and sand. Finally solid 

 parallel lines of masonry may be constructed, which are no other 

 than dams running parallel to the stream, and united to the old 

 banks by wings, and may be looked upon as artificial banks to 

 the stream. 



The top of these dams must then be of about the average level 

 of the stream so that all flood-water passes over it, carrying with 

 it silt and gravel which gradually fills-up the site of the slack 

 water. Sometimes where there is an extensive track of slack 

 water, it may be covered with a net-work of dams crossing one 

 anotiier, which gets filled with silt, &c. ; if these dams are 

 gradually raised as the spaces between them become filled, the 

 slack-water may entirely disappear, and the lateral dams be no 

 longer overflowed at high water. 



